S186: ACQUIRED RESISTANCE TO BISPECIFIC ANTIBODIES BY GENETIC OR EPIGENETIC INACTIVATION OF THE TARGET IN MULTIPLE MYELOMA
Authors: Jennifer Derrien, Sarah Gastineau, Antoine Frigout, Nils Giordano, Mia Cherkaoui, Victor Gaborit, Elise Douillard, Magali Devic, Florence Magrangeas, Philippe Moreau, Stéphane Minvielle, Cyrille Touzeau, Eric Letouzé
Journal: HemaSphere (2023)

Background: Bispecific antibodies (BsAb) are monoclonal antibodies that redirect T cells by targeting both the T cell co-receptor CD3 and markers expressed on the surface of tumor cells. BsAb targeting BCMA (teclistamab) or GPRC5D (talquetamab) demonstrated promising efficacy in relapsed or refractory multiple myeloma (RRMM) in recent phase 1-2 studies. However, ~30% of patients did not respond, and half of the responders experienced disease progression within 12 months. Thus, it is crucial to understand the mechanisms of innate and acquired resistance to BsAb. Aims: Here, we explore the mechanisms of acquired BsAb resistance in two RRMM patients enrolled in phase 1 talquetamab trials. Methods: We conducted a multi-omic characterization of myeloma cells, before talquetamab treatment (t1) and after relapse (t2), by whole genome sequencing (WGS) and single-cell Multiome allowing simultaneous profiling of gene expression (RNA-seq) and open chromatin (ATAC-seq) from the same cells. Results: MM-01-0288 was a female patient diagnosed with immunoglobulin G (IgG) myeloma at 64 yo. After 4 previous therapy lines, she received talquetamab in combination with carfilzomib, and achieved very good partial reponse for 6 months before relapse. WGS at t1 revealed clonal t(4;14), 1q gain, 13q loss and a focal deletion at 12p encompassing GPRC5D locus, so that a single copy of the BsAb target remained in tumor cells. Other driver alterations included KRAS G12R mutation and bi-allelic RB1 and CDKN1B inactivation (Fig. 1A). Substantial genetic evolution was observed at t2, including the emergence of 7 resistant subclones harboring damaging GPRC5D alterations: 3 frameshift indels (E27fs, S125fs, F158fs), 2 nonsense mutations (W217*, W237*), an in-frame deletion of 4 amino acids (G97-F100), and a 10 kb deletion encompassing the transcription start site (Fig. 1B). Thus, talquetamab resistance in this patient with a pre-existing 12p deletion involved the independent acquisition of second hits in GPRC5D by several subclones, leading to the bi-allelic inactivation of the target. MM-01-0221 was a female patient diagnosed with light chain myeloma at 67 yo. After 5 therapy lines in 6.5 years, she received talquetamab and achieved partial response for 5 months before relapse (Fig. 1C). WGS revealed a clonal t(11;14) and bi-allelic TP53 inactivation, but no alteration of GPRC5D locus in the relapse. By contrast, RNA-seq revealed a sharp decrease of GPRC5D expression between t1 (210 transcripts per million, tpm) and t2 (1.3 tpm, fold-change=0.0062, Fig. 1D). In absence of genetic insult, we examined the chromatin landscape at GPRC5D locus. We identified the loss of 70 chromatin accessibility peaks in an extended 1.3 Mb region, including GPRC5D promoter, 4 referenced enhancers and 21 other peaks found to interact with GPRC5D promoter in conformational Hi-C data (Fig. 1E). Thus, acquired talquetamab resistance in this patient involved long-range epigenetic silencing of GPRC5D locus, with coordinated chromatin silencing of its promoter and enhancer regions. Summary/Conclusion: In conclusion, we report two mechanisms of acquired talquetamab resistance resulting from genetic or epigenetic inactivation of the target. Pre-existing chromosome 12p deletion may favor the emergence of resistant subclones with inactivation of the second GPRC5D allele, mirroring previous results on BCMA CAR-T resistance. We anticipate that a thorough monitoring of the genetic and epigenetic status of the targets will help guiding the choice of BsAb and early detection of resistance in MM.Keywords: Drug resistance, Genomics, Multiple myeloma, Immunotherapy